The present invention relates to lithium metal oxides for use as positive electrode materials for lithium and lithium-ion secondary batteries, and to methods of making lithium metal oxides.
Lithium metal oxides of the formula LiMO2, wherein M is a transition metal, are important cathode (positive electrode) materials for rechargeable lithium and lithium-ion batteries. Examples of LiMO2 compounds include LiCoO2, LiNiO2, and LiMnO2. Presently, LiCoO2 is used in most commercial lithium and lithium-ion batteries as a cathode material.
LiMO2 compounds can have different crystal structures and phases, even within the same compound. For example, LiCoO2 synthesized at greater than 700xc2x0 C. has a hexagonal layered structure analogous to xcex1-NaFeO2. LiCoO2 synthesized at around 400xc2x0 C., however, has a cubic spinel-like structure analogous to Li2Ti2O4. Both structures have essentially the same FCC (face centered cubic) closed packed arrangement for oxygen except the layered structure has a small distortion in the direction perpendicular to the layers. Additionally, the two structures differ in cation arrangement.
It has been determined that the cubic spinel-like LiCoO2 turns into hexagonal layered LiCoO2 when heated to temperatures above 700xc2x0 C. Therefore, phase transformation between the two structures is possible and the layered structure is energetically favored only at high temperatures. Layered LiCoO2 also has an energetically favored tendency of changing into spinel LiCo2O4 when 50% of the lithium ions are removed from the LiCoO2 during electrochemical charging. See A. van der Ven et al., Phys, Rev. B 58, 2975 (1998); and H. Wang et al., J. Electrochem. Soc., 146, 473 (1999). The spinel-like LiCoO2 and spinel LiCo2O4 also have essentially the same atom arrangement except that lithium is at the octahedral 16c site in spinel-like LiCoO2 and at tetrahedral 8a site in spinel LiCo2O4.
The tendency of the phase transformation from hexagonal layered LiMO2 to cubic spinel-like LiMO2 is not unique to LiCoO2. Layered LiMnO2 also turns into spinel-like LiMnO2 only after a few cycles in an electrochemical cell. Although a cubic spinel-like LiNiO2 has not been experimentally observed, Li0.5NiO2 (50% delithiated LiNiO2) will indeed turn into LiNi2O4 spinel.
The electrochemical performance of LiMO2 compounds having a cubic spinel-like structure has been found to be particularly poor, especially compared to layered structures. Moreover, the mere presence of the cubic spinel-like structural phase within the layered phase or on the surface of the layered phase has also been found to be detrimental to battery performance. In particular, the presence of cubic spinel-like phases within the layered crystal structure impedes the diffusion of lithium ions during the charge and discharge cycles of the rechargeable lithium or lithium-ion battery. Furthermore, because the cubic spinel-like phase is energetically favored and only kinetic limitations prevent large scale phase transformation, the presence of localized cubic spinel-like structures can act as a seed for phase transformation to readily occur in the LiMO2 compound. Therefore, even the minor presence of cubic spinel-like phases, even at levels that cannot be detected by bulk techniques, such as powder x-ray diffraction (XRD), can cause problems in battery cycling.
The present invention provides lithium metal oxides that are substantially single-phase compounds having hexagonal layered crystal structures that are substantially free of localized cubic spinel-like structural phases. Therefore, the lithium metal oxides of the invention have more consistent electrochemical performance than prior art compounds. In addition, the lithium metal oxide compounds of the invention have good structural stability and maintain their structure through cycling. Therefore, the lithium metal oxides of the invention are useful for rechargeable lithium and lithium ion secondary batteries.
The lithium metal oxides of the invention have the formula Lixcex1Mxcex2Axcex3O2, wherein M is one or more transition metals, A is one or more dopants having an average oxidation state N such that +2.5 less than N less than +3.5, 0.90xe2x89xa6xcex1xe2x89xa61.10 and xcex2+xcex3=1. As measured using powder x-ray diffraction, the Lixcex1Mxcex2Axcex3O2 compounds according to the invention preferably have no diffraction peaks at a smaller scattering angle than the diffraction peak corresponding to Miller indices (003). In addition, the ratio of the integrated intensity of the diffraction peak corresponding to Miller indices (110) to the integrated intensity of the diffraction peak corresponding to Miller indices (108) using powder x-ray diffraction is preferably greater than or equal to 0.7, more preferably greater than or equal to 0.8. The ratio of the integrated intensity of the diffraction peak corresponding to Miller indices (102) to the integrated intensity of the diffraction peak corresponding to Miller indices (006) using powder x-ray diffraction is preferably greater than or equal to 1.0, more preferably greater than or equal to 1.2. The average oxidation state of the dopants N is preferably about +3.
In one preferred embodiment of the invention, the Lixcex1Mxcex2Axcex3O2 compound is LiCoO2. As measured using electron paramagnetic resonance, the LiCoO2 compounds of the invention typically have a change in intensity from the peak at about g=12 to the valley at about g=3 of greater than 1 standard weak pitch unit, and more typically of greater than 2 standard weak pitch units.
In addition to the Lixcex1Mxcex2Axcex3O2 compounds above, the present invention is also directed to the dilithiated forms of these compounds resulting from the electrochemical cycling of these compounds. Specifically, the present invention includes Lixcex1xe2x88x92xMxcex2Axcex3O2 compounds wherein 0xe2x89xa6xxe2x89xa6xcex1 that are derived by electrochemically removing x Li per formula unit from a compound having the formula Lixcex1Mxcex2Axcex3O2, wherein M is one or more transition metals, A is one or more dopants having an average oxidation state N such that +2.5xe2x89xa6Nxe2x89xa6+3.5, 0.90xe2x89xa6xcex1xe2x89xa61.10 and xcex2+xcex3=1. The Lixcex1xe2x88x92xMxcex2Axcex3O2 compounds are substantially single-phase lithium metal oxide compounds having hexagonal layered crystal structures that are substantially free of localized cubic spinel-like structural phases.
The present invention further includes lithium and lithium ion secondary batteries including a positive electrode comprising a compound having the formula Lixcex1Mxcex2Axcex3O2, wherein M is one or more transition metals, A is one or more dopants having an average oxidation state N such that +2.5xe2x89xa6Nxe2x89xa6+3.5, 0.90xe2x89xa6xcex1xe2x89xa61.10 and xcex2+xcex3=1. The Lixcex1Mxcex2Axcex3O2 compound used in the positive electrode has a substantially single phase, hexagonal layered crystal structure and is substantially free of localized cubic spinel-like structural phases.
The present invention further includes a method of preparing compounds having a substantially single phase, hexagonal layered crystal structure that are substantially free of localized cubic spinel-like structural phases. A lithium metal oxide having the formula Lixcex1Mxcex2Axcex3O2, wherein M is one or more transition metals, A is one or more dopants having an average oxidation state N such that +2.5xe2x89xa6Nxe2x89xa6+3.5, 0.90xe2x89xa6xcex1 less than 1.10 and xcex2+xcex3=1, is provided at a temperature of at least about 600xc2x0 C., and preferably of greater than 800xc2x0 C. The lithium metal oxide is then cooled at a rate of greater than 8xc2x0 C./min, preferably between 8xc2x0 C./min and 140xc2x0 C./min, more preferably between 10xc2x0 C./min and 100xc2x0 C./min. The lithium metal oxide can be synthesized at a temperature of at least about 600xc2x0 C., and preferably of greater than 800xc2x0 C., and then cooled at these rates, or the lithium metal oxide can be previously synthesized, heated to a temperature of at least about 600xc2x0 C., and preferably of greater than 800xc2x0 C., and then cooled at these rates. The lithium metal oxide is preferably uniformly cooled to provide homogeneity throughout the material being produced.
In a preferred method embodiment of the invention, the Lixcex1Mxcex2Axcex3O2 compound is LiCoO2 and is prepared by the method of the invention using a lithium source compound and a cobalt source compound. In particular, the preferred lithium source compound is selected from the group consisting of Li2CO3 and LiOH and the preferred cobalt source compound is selected from the group consisting of Co3O4 and Co(OH)2. More preferably, the LiCoO2 is prepared from Li2CO3 and Co3O4.
These and other features and advantages of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description and accompanying drawings, which describe both the preferred and alternative embodiments of the present invention.